As the number of subscribers for video-on-demand (VOD) services continues to grow, consistent and dependable delivery of such services becomes critical. One common mechanism for ensuring that data transmissions are not interrupted by server failure is known in the art as “failover.” Failover refers generally to the technique of automatically switching to a backup server when a primary server fails. Failover is an important fault-tolerance feature of systems that must be constantly available such as email systems and database servers.
Systems with failover capability typically group servers into failover pairs. Each failover pair includes a primary server that is active and a secondary server that is brought online only when a failover occurs. Other designations used interchangeably to refer to the primary and secondary servers are master and slave, active and standby, and primary and backup.
Although vendors often assert that their failover systems provide “instant” or “immediate” failover, these systems typically initiate a failover seconds or even minutes after a failure occurs. For many types of applications this delay is acceptable. For example, a delay of several seconds will typically go unnoticed by users accessing a database or email server due to the non-time-sensitive nature of the data being accessed. But such delays cannot be tolerated by providers of premium digital-media services.
For example, suppose a customer accesses a VOD service provided by a local cable television provider. The cable television provider will typically employ a plurality of digital media servers to provide VOD services to its customers. If any VOD server currently delivering content experiences a catastrophic failure and goes offline, the movie is disrupted and the customer's movie viewing experience is unsatisfactory.
Moreover, due to the enormous storage requirements and time-sensitive nature of delivering digital media, the process of failover for a digital media server is far more complex to implement than that of a database or Web server. The growth of VOD, subscription VOD (SVOD), and everything-on-demand (EOD) services, combined with the growing subscriber density served by each digital media server, requires stricter fault tolerance levels for asset availability.